The present invention relates, in general, to a procedure for judging the quality of printed sheets. More particularly, the invention relates to a method for judging the quality of printed images on a printing carrier, such as paper, wherein a master image is obtained by first electronically scanning a master printed sheet to determine an average grey value for each of a number of image elements, and thereafter scanning a multiplicity of proof sheets that have been judged to be acceptable by a human inspector to establish a subjective tolerance range. Subsequently, printed sheets are electronically reviewed and the grey values for selected image elements are compared with the stored values for those elements. Sheets with grey values within the allowable tolerance ranges are judged acceptable, while those with values outside the master tolerance ranges are rejected.
Devices for judging the quality of printed sheets or proof copies, are generally known; see, for example, European published unexamined patent application No. 0194331. In general, the checking of print quality refers to the measuring of ink density values in measuring fields or areas selected for that purpose. Register deviations are checked by measuring special register faults and contrast marks for analyzing dot increases and contrast. Initially, these quality control checks could not be carried out over an entire printed sheet, but systems became available on the market which permitted such monitoring with the help of image exposure devices which projected a printed sheet or a section of a printed sheet as a standing image on a monitor. Such standing images were stored in an image memory so that the printer could look at a selected still image of a printed sheet and could control the printed product on-line. Once it was known to do this, it became possible to compare later images derived from the printed products with the stored initial image and to produce signals representing any deviations, and such systems are now known.
However, it has been found such image comparison systems can result in excessive losses, for such systems operate to produce error signals whenever an image element comparison exceeds a predetermined error limit, represented by a defined difference between nominal and actual values. Such a rigid system of detecting errors is unusable in practice, since an exact correspondence of individual image elements between the "master" image and a printed image cannot be guaranteed during a printing run because of a variety of factors such as deformation of the print carrier, and the like. For example, in intaglio printing a line on the image to be judged may move with respect to its nominal position on the printing carrier, which may be a sheet or web, from one copy to the next during a production run. Such displacements of printed image elements can cause error signals in an automatic system, even though the differences between the master image and the sample image are small, and would be judged to be acceptable by an inspector. Another example is the measurement of ink density in a sample, where the average density, or grey value, of the image within a designated region, or image element, is measured. The grey value may vary from a nominal value by an amount which would produce an error signal in an automatic system, but which would be acceptable for an inspector.
A difficulty with prior automatic image comparison systems, therefore, is that they may reject printed images in cases where an inspector might consider the differences to be insufficient to cause an error signal. Tests have shown that visual examination of a printed image for the detection of printing errors is very subjective. When examining homogeneous surfaces; for example, unprinted sections, the human eye acts with the highest sensitivity to detect errors, and in such a situation, the slightest irregularities will be recognized immediately. On the other hand, the same irregularities within a printed section may not be recognized by a human inspector. However, both irregularities would cause an automatic inspection system to reject the printed image. Thus, there are significant limits to the use of automatic error detection systems for measuring irregularities in a printed product.